Arms purchases from the US are central to the Saudi kingdom's strategy of asserting its military leadership in the Gulf and confronting Iranian influence. This US deal includes significant offensive capabilities – thus the repeated warnings from Tehran about it being "destabilising".

In public the Saudis and their partners in the Gulf Cooperation Council support using diplomatic means to tackle Iran's alleged nuclear ambitions, but express greater concern behind closed doors, diplomats say. Iran insists it seeks only civilian nuclear power, not weapons.

Relations between Washington and Riyadh were badly damaged by the 9/11 attacks and the identification of the Saudi origins of Osama bin Laden and al-Qaida. But common strategic interests and pressures generated by the arms industry and the recession helped smooth differences. US defence sales to the Gulf doubled from $19bn in 2001-2004 to $40bn in 2005-2008.

It is striking that this deal has met little opposition from the pro-Israeli lobbies, which in the past have worked to prevent the Saudis acquiring advanced equipment. Nowadays Saudi Arabia also supports the Arab peace initiative, which offers to recognise Israel in exchange for a Palestinian state. King Abdullah (with Barack Obama, right) is said to have been convinced of the sincerity of the US commitment to Riyadh and pushed the arms request despite his air force wanting to divide sales between the US and Europe. Recent months have seen mounting tensions between Riyadh and Tehran, with Arab regimes pressing the US to adopt a tougher stance. In a meeting with Hillary Clinton in Riyadh in February, foreign minister Saud al-Faisal stated that the Iranian nuclear threat demands "a more immediate solution" than sanctions. The package reflects the convergence of the strategic views of the US, Israel and conservative Arab states regarding Iran's alleged nuclear ambitions and bid for regional influence. But Saudi Arabia denies reports it has secretly agreed to allow Israeli planes through its airspace if they were sent to bomb nuclear sites in Iran.